Solutions outlined here: http://www.zimbra.com/forums/announc...-16-prior.html
Check the Announcements forums and the sticky in this (Administrators) forum. The version of ClamAV (0.94) in ZCS 5.0.9 and before has been EOL'd and will no longer work. You need to either upgrade your ZCS installation or do an in-place upgrade of just the ClamAV portion.
Follow Mike's links. Another thread from Mike is also at:
This provides a bunch of solutions for you whether you decide to upgrade Zimbra, Clam or decide to temporarily disable it.
The simplest possible solution, and one that has often worked for me, is in this thread: